1. Field of Invention
The present invention relates to an analogue buffer. More particularly, the present invention relates to a source-follow type analogue buffer using poly-Si TFTs for an active matrix display.
2. Description of Related Art
Low temperature poly-Si (LTPS) thin film transistors (TFTs) allow for peripheral integration of driving circuits with a pixel panel of an active matrix display due to a high current driving capability. However, it is well known that the integration of whole driving circuit with poly-Si TFTs is very difficult due to the rather poor characteristics and non-uniformity of poly-Si TFTs compared with single crystal Si large scale integrated circuits (LSIs). Among the driving circuits using poly-Si TFTs, analogue buffers are indispensable to drive the load capacitance of the data bus in the panel. Source follower is considered an excellent candidate for the analogue buffer circuit for the “System on Panel (SOP)” application because of its simplicity and low power dissipation.
A typical source follower 100 using a LTPS TFT in an active matrix display is shown in FIG. 1A. The gate of the TFT 110 in the source follower 100 coupled to a input voltage Vin and the drain of the TFT 110 is coupled to an operation voltage Vdd. The source of the TFT 110 is coupled to ground through a load capacitor (Cload). The waveform of output voltage Vout of the source follower 100 is depicted in FIG. 1B. It is observed that the final output voltage Vout is not kept constant, but exceeds the value of Vin-Vth expected in principle, where the Vth is a threshold voltage of the TFT 110. It is ascribed to the sub-threshold current. As shown in FIG. 1C, which depicts drain current(ID) and the voltage between gate and source of the TFT 110 (VGS) curves, the sub-threshold swing of LTPS TFTs is about 0.3V/dec which is much larger than that of a metal-oxide-semiconductor field effect transistor (MOSFET) (0.06V/dec). Consequently, the typical source follower 100, as an analogue buffer for active matrix display, will be sensitive to the charging time for various product specifications such as frame rates for the active matrix displays and can not have a constant output voltage.
A further conventional source follower using a poly-Si TFT in a liquid crystal display is shown in FIG. 2A. The source follower 200 includes TFTs M1 and M2, a capacitor C1 and a plurality of switches S1˜S4. Node N1, coupled to an input voltage Vin through the switch S1, is connected to node N2 under control of the switch S2 and also connected to a gate of the TFT M1. Node N2 is connected to node N3 under control of the switch S3 and is further connected to node N4. Node N3 is connected to one terminal of the capacitor C1 and a gate terminal of the TFT M2. Node N4 is connected to a data line under control of the switch S4. The voltage level of the node N4 is an output voltage Vout of the source follower 200. A source of the TFT M1 is connected to the ground and the drain of the TFT M1 is connected to node N4, the output terminal. The TFT M2 is a PMOS transistor and its drain is connected to an operation voltage Vdd and its source is connected to the node N4.
Refer to FIG. 2B, which shows a relationship between the input voltage Vin and the output voltage Vout as denoted by the reference number 210. In a perfect case for the source follower, the output voltage Vout should be the same as the input voltage Vin. However, an error voltage which is the difference between the input voltage Vin and the output voltage Vout exists in a practical case. As denoted by the reference number 220, it shows that when the input voltage Vin is increased, the output voltage Vout is not the same as the input voltage Vin and the error voltage is floating from about 80 mV to about 175 mV if the input Vin is changed from 2.5V to 8V. If an output voltage of the source follower is large for driving in the display, for example, 10V, the error voltage may not cause serious influence on the driving operation. However, if the output voltage of the source follower is small for driving in the display voltage, for example, 0.5V˜2V, the error voltage may be larger than one gray scale voltage, which will cause serious influence on the display quality.